Lithium ion secondary batteries have the characteristics of being small, lightweight, and high in energy density. Such batteries can also be repeatedly charged and discharged and are used for a variety of purposes. Therefore, in recent years, improvements to battery members such as electrodes have been examined in order to further increase the performance of lithium ion secondary batteries.
An electrode for a lithium ion secondary battery generally includes a current collector and an electrode mixed material layer formed on the current collector. The electrode mixed material layer is formed by, for example, applying a slurry composition onto the current collector and drying the slurry composition. The slurry composition is formed by, for example, dispersing an electrode active material and a binder in a dispersion medium, also blending in a conductive material and the like as necessary. In order to further increase the performance of lithium ion secondary batteries, in recent years, attempts have been made to improve each of the materials blended into the slurry composition used to form the electrode mixed material layer.
For example, regarding a slurry composition for a negative electrode used to form the negative electrode for a lithium ion secondary battery, it has been proposed to increase the performance of the lithium ion secondary battery by improving the binder. Specifically, for example JP2010-40228A (PTL 1) proposes a slurry composition for a negative electrode that uses a binder including a polymer latex (a) that has a number average particle size of 80 nm to 120 nm, a glass transition temperature of 5° C. to 50° C., and a toluene gel content of 70% or more, and a polymer latex (b) that has a number average particle size of 150 nm to 280 nm, a glass transition temperature of −50° C. to 0° C., and a toluene gel content of 70% or more. JP2000-67871A (PTL 2), for example, proposes a slurry composition for a negative electrode that uses a binder including (a) a styrene-butadiene copolymer with a bound styrene content of 20% by mass or more to 70% by mass or less and (b) a styrene-butadiene copolymer with a bound styrene content of 80% by mass or more to less than 100% by mass.